1. Field of the Invention
The present invention relates to a carbon electrode. More particularly, the present invention is concerned with a carbon electrode having low polarizability which is useful as an anode to be employed in the electrolysis of a molten salt system containing a fluoride, and a method for producing the same. The present invention also relates to a method for producing fluorine by the electrolysis of an electrolyte comprising a mixed molten salt system of potassium fluoride and hydrogen fluoride, the electrolysis being conducted in an electrolytic cell using as an anode this novel electrode having low polarizability.
2. Discussion of Related Art
In the electrolysis of a molten salt electrolyte comprising a fluoride by the use of a carbon electrode as an anode, there is generally observed occurrence of the so-called anode effect which is represented by an abrupt spontaneous rise of voltage and decrease of current due to the anodic polarization. In practicing the conventional method of electrolyzing a molten salt electrolyte comprising a fluoride, various difficulties have been encountered due to the occurrence of such anode effect.
With respect to the electrolytic production of fluorine by using a molten salt electrolyte of a KF-2HF system and a carbon anode, the operation is conducted, for example, under such conditions that an electrolytic cell of 5000 A scale is employed and the electrolysis is conducted at an anodic current density of 10 to 13 A/dm.sup.2. Under such a load, the cell voltage is 10 V. In the case of the electrolytic production of fluorine, despite the fact that the anodic current density is as small as one-fifth that exhibited in the electrolysis of brine, the cell voltage is as high as 2.3 times that exhibited in the electrolysis of brine. Most of this high cell voltage exhibited in the electrolytic production of fluorine is accounted for by the anodic overvoltage, and the energy of this overvoltage is dissipated as Joule heat. The reason why an anodic current density exhibited in the electrolysis of a fluoride is so low is that a film of graphite fluoride having an extremely low surface energy is formed on the carbon electrode due to the reaction between the evolved fluorine and the carbon electrode during the course of the electrolysis reaction, thereby interrupting the contact between the carbon electrode and the electrolyte. This phenomenon is so-called "anode effect". The anode effect is represented by an abrupt spontaneous rise of voltage and decrease of current due to the anodic polarization. If the anode effect occurs even once, it is extremely difficult to bring the electrolysis back to the normal state, and it becomes necessary to take the electrode out of the electrolyte and polish the electrode, or to replace the electrode and the electrolyte with fresh ones. Therefore, various problems are disadvantageously encountered in operational administration, and this is one of great factors rendering it difficult to perform the electrolytic production of fluorine.
In order to obviate the above problems, there have been proposed a method in which lithium fluoride is added to an electrolyte of a KF-2HF system in an amount exceeding the solubility, and a method in which a fluoride, such as lithium fluoride, is incorporated in the molding material for a carbon block before molding and calcination (see, for example, U.S. Pat. No. 4,312,718). However, neither of the above-mentioned methods is satisfactory with respect to the suppression of the anode effect. Further, in the case of the method in which lithium fluoride is added to the electrolyte in an amount exceeding the solubility, a sludge of lithium fluoride is likely to accumulate on the bottom of the electrolytic cell. This accumulation of the sludge not only leads to an iR loss, but also to a nonuniformity in heat conduction and, thus, local differences in the temperature of the electrolytic bath, thereby adversely affecting the circulation of the electrolytic bath. Further, the sludge may clog the communicating portions in the electrolytic cell, or stick to the bottom of the cell so fixedly that the sludge cannot easily be scraped off. On the other hand, in the method in which a fluoride such as lithium fluoride is incorporated in the molding material for a carbon block, the carbon material is blended with lithium fluoride, and the resultant blend is molded and then calcined. In the resultant carbon block, the lithium fluoride is not contained in the pore phase but in the carbon matrix phase of the resultant porous carbon block. The calcination for producing a carbon block must be conducted at a temperature of from 800.degree. to 1,200.degree. C. However, lithium fluoride is not stable at a temperature of higher than about 800.degree. C. and, therefore, not only is a large portion of the lithium fluoride incorporated in the carbon matrix phase likely to be melted out and partly vavorized away during the calcination, but also the incorporation of lithium fluoride in the carbon block is likely to be non-uniform, so that the effect of the incorporation of lithium fluoride is unsatisfactory.